Flat-panel display devices, for example plasma, liquid crystal and Organic Light Emitting Diode (OLED) displays have been known for some years and are widely used in electronic devices to display information and images. Such devices employ both active-matrix and passive-matrix control schemes and can employ a plurality of light-emitting elements. The light-emitting elements are typically arranged in two-dimensional arrays with a row and a column address for each light-emitting element and having a data value associated with each light-emitting element to emit light at a brightness corresponding to the associated data value.
Active-matrix electroluminescent devices typically employ thin-film electronic components formed on the same substrate as the light-emitting elements thereof to control light emission from individual light-emitting elements thereof. Such thin film electronic components are subject to manufacturing process variabilities that may cause such components to have variable performance. In particular, the voltage at which thin-film transistors turn on (“threshold voltage”) may vary. Low-temperature polysilicon (LTPS) devices have a short-range variability due to the variability in the silicon annealing process used to form such devices. Amorphous silicon devices typically have a long-range variability due to variabilities in the silicon deposition processes. Further, threshold voltage properties of such thin-film devices may changes significantly with use over time, particularly for amorphous silicon devices. Typical large-format displays, e.g., employ hydrogenated amorphous silicon thin-film transistors (aSi-TFTs) to drive the pixels in such large-format displays. However, as described in “Threshold voltage instability of amorphous silicon thin-film transistors under constant current stress” by Jahinuzzaman et al in Applied Physics Letters 87,023502 (2005), the aSi-TFTs exhibit a metastable shift in threshold voltage when subjected to prolonged gate bias. This shift is not significant in traditional display devices such as LCDs because the current required to switch the liquid crystals in LCD display is relatively small. However, for OLED applications, much larger currents must be switched by the aSi-TFT circuits to drive the organic materials to emit light. Thus, OLED displays employing aSi-TFT circuits are expected to exhibit a significant voltage threshold shift as they are used. This voltage shift may result in decreased dynamic range and image artifacts. Moreover, the organic materials in OLED devices also deteriorate in relation to the integrated current density passed through them over time so that their efficiency drops while their resistance to current increases.
One approach to avoiding the problem of voltage threshold shift in aSi-TFT circuits is to employ circuit designs whose performance is relatively constant in the presence of such voltage shifts. For example, US20050269959 A1 entitled “Pixel circuit, active matrix apparatus and display apparatus” describes a pixel circuit having a function of compensating for characteristic variation of an electro-optical element and threshold voltage variation of a transistor. The pixel circuit includes an electro-optical element, a holding capacitor, and five N-channel thin film transistors including a sampling transistor, a drive transistor, a switching transistor, and first and second detection transistors. Alternative circuit designs employ current-mirror driving circuits or voltage to current conversion circuits, that reduce susceptibility to transistor performance, e.g., US2005/0180083, US2005/0024352 and WO2006/012028. Other methods, such as taught in US20040032382, WO2005/015530, and WO2006/046196, employ photo-sensors in pixel-driving circuits and employ feedback control so that pixels emit a desired amount of light regardless of organic material or transistor performance. However, such designs typically require complex, larger and/or slower circuits than the two-transistor, single capacitor circuits otherwise employed, thereby increasing costs and reducing the area on a display available for emitting light and decreasing the display lifetime.
Other compensation methods are described in the prior art to mitigate the effects of changing organic material properties and changing thin-film transistor properties. One group of compensation methods attempts to prevent the problem from occurring, for example by employing reverse-biasing to undo thin-film circuit changes. For example, US20040001037 A1 entitled “Organic light-emitting diode display” describes a technique to reduce the rate of increase in threshold voltage, i.e. degradation, of an amorphous silicon TFT driving an OLED. A first supply voltage is supplied to a drain of the TFT when a first control voltage is applied to a gate of the TFT to activate the TFT and drive the OLED. However, a second, lower supply voltage is supplied to the drain of the TFT when a second control voltage is applied to the gate of the TFT to deactivate the TFT and turn off the OLED, whereby a voltage differential between the drain and the source when the second control voltage is applied to the gate is substantially lower said first supply voltage. This reduces degradation of the TFT. However, such schemes typically require complex additional circuitry and timing signals, thereby reducing the area on a display available for emitting light and decreasing the display lifetime and cost. Alternatively, by increasing the size of organic light-emitting elements or placing a maximum on the current that passes through the organic elements, degradation may be decreased. However, these methods have limited utility in that the degradation problem is not solved but rather reduced.
Other techniques employ external compensation to mitigate the effects of changes in the display device. For example, U.S. Pat. No. 6,995,519 describes an organic light emitting diode (OLED) display comprising an array of OLED display light-emitting elements, each OLED display light-emitting element having two terminals; a voltage sensing circuit for each OLED display light-emitting element in the display array including a transistor in each circuit connected to one of the terminals of a corresponding OLED display light-emitting element for sensing the voltage across the OLED display light-emitting element to produce feedback signals representing the voltage across the OLED display light-emitting elements in the display array; and a controller responsive to the feedback signals for calculating a correction signal for each OLED display light-emitting element and applying the correction signal to data used to drive each OLED display light-emitting element to compensate for the changes in the output of each OLED display light-emitting element. However, this design also suffers from the need for additional circuitry in each active-matrix pixel.
It is known in the prior art to measure the performance of each pixel in a display and then to correct for the performance of the pixel to provide a more uniform output across the display. U.S. Pat. No. 6,081,073 entitled “Matrix Display with Matched Solid-State Pixels” by Salam granted Jun. 27, 2000 describes a display matrix with a process and control means for reducing brightness variations in the pixels. This patent describes the use of a linear scaling method for each pixel based on a ratio between the brightness of the weakest pixel in the display and the brightness of each pixel. However, this approach will lead to an overall reduction in the dynamic range and brightness of the display and a reduction and variation in the bit depth at which the pixels can be operated.
U.S. Pat. No. 6,473,065 B1 entitled “Methods of improving display uniformity of organic light emitting displays by calibrating individual pixel” by Fan issued 20021029 describes methods of improving the display uniformity of an OLED. In order to improve the display uniformity of an OLED, the display characteristics of all organic-light-emitting-elements are measured, and calibration parameters for each organic-light-emitting-element are obtained from the measured display characteristics of the corresponding organic-light-emitting-element. The calibration parameters of each organic-light-emitting-element are stored in a calibration memory. The technique uses a combination of look-up tables and calculation circuitry to implement uniformity correction. However, the described approaches require either a lookup table providing a complete characterization for each pixel, or extensive computational circuitry within a device controller. This is likely to be expensive and impractical in most applications.
Copending, commonly assigned U.S. Ser. No. 11/093,115 describes a method for the correction of average brightness or brightness uniformity variations in OLED displays wherein the brightness of each light-emitting element is measured at two or more, but fewer than all possible, different input signal values. While brightness or luminance measurements may be practical in a manufacturing environment, and thus appropriate for initial display calibration, they may be problematic after the display is subsequently put into use and thus less practical for performance of aging compensation.
US2006/0007249 discloses a method for operating and individually controlling the luminance of each pixel in an emissive active-matrix display device including storing transformation between digital image gray level value and display drive signal that generates luminance from pixel corresponding to digital gray level value; identifying target gray level value for particular pixel; generating display drive signal corresponding to identified target gray level based on stored transformation and driving particular pixel with drive signal during first display frame; measuring parameter representative of actual measured luminance of particular pixel at a second time after the first time; determining difference between identified target luminance and actual measured luminance; modifying stored transformation for particular pixel based on determined difference; and storing and using modified transformation for generating display drive signal for particular pixel during frame time following first frame time.
WO 2005/057544 describes a video data signal correction system for video data signals addressing active matrix electroluminescent display devices wherein an updated electrical characteristic parameter X is calculated for each drive transistor by measuring actual current through a power line in comparison to expected current determined using a model and a previously stored parameter value, where subsequent video data signals are corrected in accordance with the calculated parameter X. Calculation of characteristic parameters based on assumed pre-determined performance relationships, however, may require consideration of many parameters having complex interactive relationships, and further may not accurately reflect actual device performance.
US 2004/0150590 describes an OLED display comprising a plurality of light emitting elements divided into two or more groups, the light emitting elements having an output that changes with time or use; a current measuring device for sensing the total current used by the display to produce a current signal; and a controller for simultaneously activating all of the light emitting elements in a group and responsive to the current signal for calculating a correction signal for the light emitting elements in the group and applying the correction signal to input image signals to produce corrected input image signals that compensate for the changes in the output of the light emitting elements of the group. While this technique is useful and effective, the problem of measuring the currents while the display is in use without causing the user to perceive luminance discontinuities, or other objectionable display artifacts necessary for performing the measurements, remains.
There is a need, therefore, for an improved method of measuring and compensating for changes in the performance of light-emitting elements in an OLED display device.